1. Field of the Invention
This invention relates to heat exchangers and mounting structures therefor. In one of its aspects, the invention relates to a mounting structure for a ceramic rotary heat exchanger.
2. State of the Prior Art
Incineration of combustible fumes is a problem currently facing many industries such as lithography, smoke houses, paint shops, etc. It has been found that combustion of these fumes at the present time is the most effective way to eliminate the fumes from these industrial processes although the cost of fuel for such incineration has been considerable. Efforts have been made to more completely recoup the heat enegy required to raise the fume and air mixture to incineration temperatures. For example, see U.S. Pat. No. 3,706,455 to Charles B. Gentry for FUME INCINERATION, and U.S. Pat. No. 3,670,667 to William A. Phillips for INCINERATION WITH EXTENDED HEAT EXCHANGE SURFACE.
It has been proposed to use a rotary heat exchanger to recover the heat from hot exhaust gases for preheating the fume-containing gases which are to be incinerated. Rotary regenerators have been used for many years for recuperating heat from hot gases. In recent years, special ceramic materials have been developed for use in these rotary regenerators. The ceramic materials have a very low coefficient of expansion and find particular utility in the automotive field in connection with turbine engines. An example of such ceramic material is Cer-Vit material which is manufactured and sold by Owens-Illinois of Toledo, Ohio.
Some of the ceramic heat exchanger structures have been of the radial flow type in that the gases flow radially through a cylindrical body. Thus, the passages extend radially through the cylindrical body. Such a heat exchanger is disclosed in the U.S. Pat. No. 2,598,182 to Kolb. In the Kolb heat exchanger structure, the cylindrical heat exchanger is supported at the edges thereof by annular bearings. The bearings are provided both on the "hot side" and the "cold side" of the heat exchanger and provisions can be made for circulating cooling air through chambers adjacent to the bearing structure. The bearing structure is metallic and has a greater coefficient of expansion than does the ceramic heat exchanger material. Accordingly, thermal stresses of a radical nature may be experienced by the relatively thermally stable ceramic core due to thermal expansion of the bearing members.
In other types of rotary heat exchangers, the gases pass axially through the ceramic core. The heat exchanger wheel is mounted for rotation about a central axis. In some cases the heat exchanger is supported at the peripheral face thereof and the rotational forces are applied to the peripheral face of the heat exchanger. Such a drive mechanism is disclosed in the U.S. Pat. No. 3,456,518 to Topouzian. In such drive mechanisms, sealing of the periphery of the heat exchanger may be a problem.
MOre conventionally, it is desirable to support the ceramic core in a central portion and apply the drive to the central core mounting. However, differences in thermal expansion characteristics of the mounting structure and the ceramic heat exchanger wheel may cause failure of the wheel. Whereas the ceramic material is relatively strong under compression, it is realtively weak in tension. In order to obtain the required strength to mount and rotate the wheel, strong metals, such as steel, are used to mount the heat exchanger wheel. If the heat exchanger wheels are conventionally mounted on the metal mounting shafts, expansion of the metal mounting shafts due to heat may cause cracking and failure of the ceramic heat exchanger wheel.
Another problem with mounting the ceramic heat exchanger on a central support is that the bearings which support the rod usually have a relatively short life. Conventionally, bearings have been mounted on each side of the heat exchanger. The bearing on the "hot side" of the heat exchanger is subject to seizing as a result of the intense heat that builds up in the bearing. This is especially true for heat exchangers for hotter gases as in incinerators, for example, where the temperature may reach 1600.degree.F.
Bracken, Jr. et al in U.S. Pat. No. 3,476,173 discloses a rotary ceramic heat exchanger wheel mounted on a central support shaft which is journaled in a bearing structure including a pair of bearings mounted at the cool side of the heat exchanger. The wheel mounting structure includes a hub which is driven by the central shaft with the hub having a plurality of fingers extending into recesses in the central part of the ceramic heat exchanger wheel. The fingers are secured to pads within the recesses to evenly distribute the load between the hub and the heat exchanger wheel. Clearance is maintained between the fingers of the hub and the ceramic wheel with the pads filling the space between the fingers and the wheel. Although the bearings are positioned on the cool side of the heat exchanger wheel, they still may have a tendency to overheat.